Pulse system



Aug, 16, 1949.

Filed Aug. 4, 1945 C. W. HANSELL.

PULSE SYSTEM Pane@ PAQ/a 79,45/14/7-75# Z/A/E of? M11/5 fami 5 Sheets-Sheet l BY Mw ATTORNEY Aug. 16, 1949. c. w. HANSELL 5 Sheets-Sheet 2 Filed Aug.4 4, 1945 CMA/NEL 3 l INVENTOR.

(2 Aff/vc; WQ/vfa BY Z/mw A 7 70m/EY.

C. W, HANSEZLL` Aug. 16, 1949.

PULSE SYSTEM 5 Sheets-Shes?l I5 Filed Aug. 4, 1945 Aug. 16, 1949.

Filed Aug. 4, 1945 Tci MQW/Wmv HAMM/frias 70' fram/1x20 FPi@ Jaafa- INVENTOR A/PfA/cf Mm/Jnz.

ATTORNEY Aug- 16, 1949- c. w. Hmusl-:LLY 2,478,920

PULSE SYSTEM 'l Filed Aug. 4, 1945 5'SheeLS-Sheet 5 Patented ug. 126V, 1949 PULSE SYSTEM Clarence` W. Hansell, Port Jeerson, N. Y., asf' signor to Radio Corporation of America, acer-jy poration of Delaware s l K Y Application August 4, 1943, Serial No.` 497,315 I.

23 claims. (c1. 25o-9) 1 The present invention relates to improvements in radio broadcasting systems.

In employing conventional methods of communication involving the use of continuous waves, it has been found that greater and greater difficulties are encountered in obtaining good transmitter and receiver frequency stability as higher and higher frequencies are utilized. When it is attempted to carry conventional methods up to frequencies exceeding` say 150 megacycles, it is found that the receiver circuits preceding the :final detector must be very broad if the received signal is to be kept within the receiver pass band. :Since very high` radio frequencies are limited in :range to not mu-ch Ymore than optical distances, equipment to utilize them must be inexpensive or it cannot compete with other forms of communication for many applications. yTherefore, the expenditure required to apply frequency control of extreme precision cannot be justiiied in such cases.

Because receiver passibands oi conventional receivers must be made. very broad, to allow for frequency variations of transmitters and receivers, a relatively great amount of noise is admitted. The noise, in turn, either limits the communication range or requires use of far greater transmitter power than would be required if frequency stability were not a problem.

Long study and development of means toobtain frequency stability has indicated that this line of attack has 'long since entered the phase of rapidly diminishing returns per unit of development and construction cost. Therefore, some other approach to the problem has been indicated.

The present invention overcomes the lforegoing difiiculties by applying pulsing methods to broadcast communication at the extremely high radio frequencies. In the pulsing communication system of the invention, 'only a relatively narrow modulation frequency band is required, and the transmitter` power isradiated in relatively short, Widely spaced, pulses of carrier current with power level'during pulses increased in inverse proportion to the percentage-time occupied by the transmitted pulses. If the pulses occupy say 1% of the total time, then the power radiated during pulses may be 100 times the amount of power obtainable continuously from continuous wave equipment' of comparable size and cost. The pulses are received withra receiver having means to make itself 'responsive to received power only during time periods when transmittedpulses are due to arrive.- This may be accomplished iby syn- Y Y 2 chronizing means similar in function to those already developed for television. In this way the eects of interference from other transmitters and of noise presentA during time periods between periods assigned to pulses of aparticular transmitter may be eliminated. "Ihus, pulse timing selectivity is substituted for lfrequency selectivity and, as a consequence, loss of signal to noise ratio due to wide receiver pass bands,inecessitated by radio carrier frequency stability, lbecomes a relatively minor factor.

In the system offtheinventiom Vmodulation Vmay be applied |by vmodulating the, amplitude,

length or timing of thek pulses. I prefer to modulate the timing of the pulses because of certain advantages possessed by v,systems employing this method, One advantage'is that the average transmitted power is substantially constant and unaffectedby the presence or absence of modulation. Incthis system, where the pulse ampli- `tude is constant, it is l, possible, by means of thresholding and limiting ampliiiers at the receiver, to greatly reduce the effects of noise so long as the amplitude of pulse currents is more than twice the peak amplitude of noise. In the receiver, the rectied,` amplied, thresholded 4.and limited received'pulses are delivered to suitable demodulating circuits.

`11n the useV of pulsingsystems, there are two methods by which short pulses may be transmitted. One is to let thetransmitter feed relatively constant power to a flywheel -circuit and then to take powerfrom rthe circuit in pulses by means of some sort of momentarily applied coupling betweenthe iiywheel circuit and the antenna. This permitsl obtaining peak pulse power nearlyl inverselyr proportional rto the percentage time occupied by theftransmitted pulses. Such Yan arrangement is described'in my copending application Serial No. 371,865, iiled December 27, 1940, now U. S. Patent 2,381,444, granted August '7, 1,945. ulllnfortunately,"the flywheel circuit and momentary coupling arrangement is at present applicable at only rather low radio frequencies and lowpulserates because, up to the present, therev are not commercially available coupling circuits and apparatus'which can be controlled as rapidly as desiredi However, there have been a number of experiments made which indicate that such a coupling canfbe provided iby further den velopment. A second method involving pulse transmission -is toapply much more than normal potentials and currents to transmitter output tubes in'shot-relatively'widely spaced pulses. By

-`= this kmeans the Yinstantaneous power dissipation in the tubes may be increased approximately in inverse proportion to the percentage pulse time..

Many types of vacuum tubes, particularly those with thoriated or oxide coated cathodes, are capable of carrying much higher electron currents than is normally permitted by heating in continuous wave service. Over a large range of potentials and currents, above 4rated values, many tubes will withstand pulse operation "at peak power outputs enormously greater than the rated continuous wave output. In general, the operating efiiciency of the tubes tends to increase with increasing potential and current because the tube impedance decreases while the load Vimpedance may be considered,constant,V As an egiample, a tube operating at anode circuit conversion efficiency and delivering an output of one watt on continuous waves vmight instead be pulsed with 10 times greater instantaneous anode current, which would result in 10 times greater output current and potential o r about l100 times greater output power. Y

The broadcasting system f the present invention employs a Vplurality f 'interconnected sound broadcasting stations each arranged to transmit the same pulse frequently, 0n the same radio carrier 'wave frequencyfbt at diierent non-overlapping timesf. 'The Vduration of "the pulses of radio frequency energy Ais made to vbe short compared to the timeY intervalsbetweenthem, so that l the different' transmissions do not coincide in time even when the timing `off pulses 'from individual transmitters is 'being modulated. 'The pulse frequency `is chosen 'to be higher than the highest modulation'frequeney, and'in thecase of speech modulation as contemplated herein, this pulse frequency might be "20,000 to 40,000 ,per second. I will assume 'ijn .the following .that .itis 25,000 per second.

By way of example assunitliat, a given Ci-ty or service area. it is desired to establish 'four radio broadcasting transmitters, eachfhandling fa different program. Assume furtherthat, if conventional amplitude modulation .methods were used, frequency instability between transmitter and receiver, plus vthe -,frequency bands occupied Aby modulation, would .rfrluirethe use of.A receiver pass bands which are 2,500,000cycles wide; According to prior broadcastingpractice, the'four transmitters would then b assigned frequency channels spaced 2,500,000 .cycles fapart and would require a total frequency Aband of 10,000,000 cycles.

'Ihe value of 2,500,000 .cycles .required band width for the receiv'erds arrived 4atas v'follqwsr In the present standard'.broadcastfband, ranging in frequency around 1,000,000 cycles persecond, transmitting stations are placed.aboutl10,`000 cycles apart. At this frequency, ithe'relative 'frequency stabilities "between transmitters .and receivers is adequate. Perhaps lewfpriced receivers may be said to `held correct .tuning within -safy plus and minus `1250 cycles or 0.125% -of `correct value, which may becolsideredgood elio-ugh, Considering that the listener .can retune occasionally. For this same .percentage accuracy in .holding correct tuning, we woul'dexpect the variations to be plus and minus 512,500 cycles .at i 10,000,000 cycles, 125,000 oyclesat ,100,000,000 cycles and 1,250,000 cyclesat1,000,000,000cycles. A'Illnerequired band .width at-1000 -megacycles weuld vthus be 2,500,000 cycles. valueof 2,500,000 .cycles as the band needed-'stille receiver t@ teen-a v1000 megacycle pulse transmitter tunedis a reasonable one and is Consistent with current experience. considering the faet that lmesacycle transmitters and receivers cannot at present benet from years of development on frequency control at the lower frequencies. If each low priced receiver must be about 2,500,000 cycles wide to keep 1000 megacycle transmitters tuned in, then the transmitters in any one area must, of course, be spaced 2,500,000 cycles apart and four such transmitters would occupy a total band of 10,000,000 cycles.

In accordance with the present invention all transmitters will be placed on the same radio carrier frequency (for example, 1000 megacycles), within some reasonable tolerance, but will share time in accordance with vthe principles of time s ldivisionmultiplex systems. Thus the transmit- Arate of say 25,000 per second, corresponding to an average time of 40 microseconds between successive pulses cf @een transmitter.- The Successively repeated time yi-,rii'ervals assigned to each tranemitter are then -a .maximum of 10 microseccnds in length, However, I propose that the allowance for frequency instabilities. They may,

V,for example, be .nominally 0.25 microsecond in length and have 'a Wave-form :is-nearly `rectangular as the `reRamillies `'7i-500.000 cycle frequency yband will permit Under these conditions pulses of .each transmitter will Aoccupy 0.625% of the time fand the pulse power yu titput of each transmiliter ymay be about i60 times the power output of a continuous wave transmitter `of equivalent Sizeand equalaveraee-nowereutnut Each transmittel" may then be modulated 1in such a manner es tosuperimpose the 'modulation upon the pulses- 1 might m0110120@ the amplitude er the. lensthvf the pulses but I prefer to modulate the timing, orc-phase, @f repetition of the pulses, while 'main- .taining .the amplitude ylength '.:of .pulses con- Stm- In the example aivenfth'etimine of pulses from feacli transmitter ymay -be modulated from the average vtime position A:by :as much `as vplus or minus 4,115 microseconds bui-gin practice, we may set a nominallimit-.of plus andrminus 3.5 microseconds, .or less, forreasons which will/be apparent later.

To receive .the waves .from each transmitter ri.10depeneently of the ethers. fin the example of the invention given, la .receiver will be used :in accordance with thje invention which is :respons ive for time periods equal ito'or somewhat less than the operating time periods assigned to each transmitter, repeated in :synchronism `with lthe pulses of aselected transmitter. During responsive periods, .received power, tfirst lheterodyned to lower frequencies, .then .frequency vband selected, .and amplifier `'threshelded and limited, will be .rectified te 2produce .directcurrents in .the vform of nearly :rectangular wave :form :pulses which are thenpassed onto ademodulator suitable `for demcdulating the type of modulation superimposedlon .the pulses. The .resulting demodulated output ,power -is .obtained at modulation frequencies 'which-may be `further:amplified and utilized in the usual Ways. Thresholding and limiting devices at the receiverare yused Lto very greatly `reduce :the effect of fall .noise 1power :having lesser amplitudes .thanfhalf .the puls peaks.

ABy .the expression pulse;.peaks is meant vthe value v.of rthe Signal pulse peaks .in the fcircuits before .thKQBhDldingI-.and limiting. It will be evident that if the threshold'is set high enough to cut 01T noise peaks, then the signal pulses must be higher `than the noise. peaks to get. through. However, when the pulses are present in the circuits they are modulated by the noise to an extent equal to the maximum of the noise peaks. The minima of the dips in pulse amplitude due to noise should not reduce the current. through the limiter. Since limiting must always beat an amplitude above the threshold amplitude, it follows that the signal current mustbe not less than twice the peak noise amplitude to obtain the full effect of thresholding and limiting.

. Compared with the conventional continuous wave amplitude modulation system of the example, such a system provides an overall signal to noise peak power ratio improvement of about 160 to l on each channel, in the receiver circuits prior to thresholding, limiting and demodulation, for a given size of transmitter apparatus, without an increase in the total frequency band required by the four transmitters. As a result of thresholding and limiting, when noise levels are not too high, the nal improvement in effective signal to noise ratio may be much more than 160 to 1. Thus, four 50 kilowatt transmitters may be made the equivalent or better in results than four 8000 kilowatt amplitude modulated transmitters by means of the invention, under the conditions assumed.

To explain and clarify the reasons for a signal to noise ratio improvement, as compared with the conventional amplitude modulation system, it may be noted that the necessity for wideningv the receiver pass band beyond the minimum required for' modulation, in order to allow for frequency in-V stabilities, results in a proportional increase'in both the r. m. s. and the peak values of combined radio frequency noise currents at the input to the demodulator of the amplitude modulation receiver. As a result, masking of the useful modulation by the noise takes place at nearly porportionally higher signal current power'levels. In other words, widening the receiver pass band raises the mean power threshold level which must beexceeded by the signal carrier current powerY before the useful modulation can come through the demodulator with small enough mutilation to be useful. This is the same kind of phenomenon as that which causes the improvement threshold noise level to rise in frequency modu-V lation receivers when the receiver pass band is widened.

In the pulsing system of the present invention, unlike frequency modulation systems, the increased noise power within the pass of the re ceiver demodulator, as the pass band is widened,- i-ss balanced by a corresponding increase in transmitter peak pulse power. Therefore, so long as the transmitted pulses are decreased in length to match the increased receiver pass band, and the pulse peak power increased to correspond, .there is no rise in the mean or average improvement threshold power level as the receiver pass band is increased. Thus, in marked contrast to frequency modulation systems, there is no increase in the minimum required average power level to work through the noise as the frequency band occupied by the system is increased.

Thresholding and limiting are fullyeffective in bringing about a great improvement in nal signal-to-noise ratio in the output of the receiverr of the present invention so long as peak noise currents do not reach above the threshold level during time intervals when the transmitter power.'

is off, 'or reach below the limiting level when the transmitter power is on. In other Words, so long as noise is not strong enough to affect the envelope of the carrier current pulse wave form, after thresholding and limiting, then the effect of all noise currents is very greatly reduced. Under these conditions the only effect of noise is to causeslight variations in the exact timing of transition from zero to maximum, or maximum to zeroiin"V the envelope wave form of current deliveredto the rectifier which converts carrier current pulses into direct current pulses in the receiver. l

It will be noted that, so long as power from the' various transmitters does not arrive at the receivers during overlapping time periods within which thev receiver is open, there can be no interference between transmitters. Even if energy from an undesired transmitter should arrive simultaneously with that of a desired transmitter while the receiver is open, no substantial interference need result so long as the combined currents from the undesired transmitter and noise do not reach half the amplitude of currents from the desired transmitter. Under this condition the thresholder and limiter suppress nearly all interference.

In practice, energy of undesired transmitters might 'occasionally arrive simultaneously withY energy of a desired transmitter due to the influence of reflections of energy over longer paths, resulting in arrival of time delayed energy at the receiver, or due to arrival of energy from distant stations operating in the same frequency bandv but located in other service areas. However, the amount of this energy will usually be much less than that from the desired transmitter so that its effect will be small.

In the practice of the invention, it is important to locate the time sharing transmitters which arein the same service area and on the same frequency as nearly as possible at the same location so that' they have nearly equal time delays to' all receivers in their service area. Otherwise, existence vof unequal time delays may result in interference, or in loss of permissible pulse time swing due to modulation.

The preferred arrangement is to load all the transmitters on the same antenna in which case it is economical to use a higher and more directive antenna which will enlarge the service area of all transmitters. This loading of all transmitters on one antenna requires the use of special cou-l pling and uncoupling circuit, operated synchro' nously with the transmitter pulses, which will be described later.

A more detailed description of the invention follows, in conjunction with a drawing, wherein:

Fig. v1 illustrates a radio broadcasting system having a plurality of interconnected broadcasting transmitting stations operating in accordance with the invention;

Fig. 2 shows the pulse timing relations of the four transmitters for the condition of no modulation;

' Fig. 3 shows the range of timing relations for the Vtransmitters in greater detail than in Fig. 2; Fig. 4'is a block diagram of a receiver for use Ain the' system;

Fig. 5'illustrates, more or less in detail, circuit arrangements corresponding to the transmitting system'of Fig. 1; ,Y Y

Fig. 6' shows the wave forms and timing rela` tions of currents in variousparts of the system of Fig. 5;

Fig. '7 illustrates in greater detail the radioV fre quency commutator arrangement of Figs. 1 an'd15.; and

Fig. 8 illustrates more or less in detail circuit arrangements which may be used for the receiver ofFife. 4.

Referring to Fig. 1, there is shown,.in block diagram, an arrangement of four pulse type radio broadcastingtransmitter stations l., 2, 3, and 4, each modulated by 'an independent sound program and all locked together so that their pulses occupy different time periods. All four transmitters are loaded on a common 'transmission line TL and associated transmitting antenna 5 through an automatic radio frequency current commutator 6 which, in effect, connects each transmitter to the line TL extending to the antenna system only while that transmitter is providing pulse power output.

The four transmitters include apparatus labeled vrespectively H, I2, I3, and lli :for converting pulsed directcurrent power to .high frequency power. These converters may beof the magnetronmscillator "type, particularly .at frequencies above l56'() megacycles or 'they may be triode oscillators or amplifiers, particularly .at frequencies below 500 megacycles. 4Each .comprises associated rectiers and pulse keyers or modulators well known vin the art, 'particularly in 'radar systems.

To determine the pulse timing, there is alpul'se frequency determining oscillator 12| 'which .is common to all transmitters and which controls the .operation of a pulse lcominutator 22 out .of

which come `pulses spaced in time according'to the desired average time spacing between pulses from the transmitters Il, i2, I3, and .l 4. Pulses from unit 22 are delivered in succession to pulse delay modulators 23, 2li, 25., and 26. There is one such pulse delay modulator .for .each .broadcasting station, and the functions of these -pulse delay modulators are to modulate the timing or phase of the output pulses from each `of transmitters il, i2, i3, and le in accordance with its own lparticular ,program modulation.

In order that there maybe provided, .at thereceiver, some means for causing the receiver to choose, automatically, the desired synchronization needed for selecting `one transmitter from the other according to predetermined receiver adjustments, the pulsing of the four transmitters is 'timed as -though there were to be five trans mitters Apulsing successively at uniform time intervals, vbut pulses of the fifth transmitter fare omitted. 'This pulse timing .relation :illustrated in Fig. 2, for the condition of n'o'modulation.

Fig. 3 .illust-rates in .more ydetail the conditions offoperationof the vpulsesin .channel 2, which are typical of all the channels. The time interval assigned to the channel, at each repetition, .is-

8 microseconds and it is assumed that :the 'transmitted pulse is 0.25 microsecond in length. The maximum degree 'of modulation of 'the'pulse corresponds to changing its timing by plus or minus 3.15 microseconds. This leaves `a time interval of 0:75 microsecond for the receiver to switch `'its gating system on or oif, for choosing pulse power of only one transmitter at a time. In practice, the `range of modulation and the available ltime interval for switching the receivermaypofzcourse, be adjusted for best results with currently available -quality :of equipment.

VFor .selectively receiving and demodulatingfthe pulses of the transmitters of the system of Eig, i1,

amplifier 33a, and demodulated in .demodulator 3e. AThe `output .from demodu-lrator 34, .at audio frequencieais then amplified in audio frequency amplifier 35 and d'elivere'dto v:loudspeaker '36.

`A portion 'of the youtput Vfrom pulse rectifier 33 isdelivered to the pulse-selector f3.1 whose function is to selecta particular 4timingrelation according to the empty time intervals :illus-V trated in ?iig. .2. Output from the .pulse selector 31 is'util-ized to synchronize `pulse Aoscillator' :.38

i which pulses :once for :each cycle Aof rotation fof the channels, lin this case, Aaccording Ato the example given, at 1a 'rate -of 254000 pulses 'per second.

-Qutput from :oscillator 38 is `passed through manually adjustable pulse delayer 3.9 and pulse length adjuster 39a by means zof which 4any .desiredone of vthe .four transmitters may be selected for reception fand'demodulation. 'Output from pulse `delayer circuit andthe pulse length adjuster Std .is used to `key :on and off the coupling from pulse rectifier 33 through kever amplier i33a to pulse Vdemodulator 3d, "to render it operative for the time .intervals occupied by only .one transmitter, excluding time *intervals occupied by 'the other transmitters.

In Figs. l and 4 someofth'e indicated parts fof the system comprise .components for performing functions which are components already known in the art and for which there are .known alternative means available. However, in yorder to facil-itate the Vpractice of the invention, I will show means for .accomplishing the .less Well known functions and give references to prior :disclosures of'suita-ble means.

In Fig. 5, I have shown asimplified circuit diagra-m of the transmitting `system which 4is :illustrated in 4the block diagram of Fig. i1. At 32| I have shown what is .sometimes called a multivibrator oscillator which yoperates Vat .a .frequency equal to the combined pulse rates ofthe yseveral transmitters plus -the `missing `pulsepof Fig. 2. .If each transmitter is 'to pulse '25,000 times persecond, and 4there are four transmitters, as illustrated, then oscillator 2l will operate at 25.,.O00X5 or 125,000 cycles per second. This multivibrator is so .designed and adjusted 4that `.pulses-.of current through each anode to -cathode'circuit of theztwo triode Velectrode structures :occupy .halffof the time. Thus, the osci-llatcriiops .from one :unbalanced condition Ato the 1other 25.05000 times per second, vremaining in reach .con-dition .for .half .of the time.

.In those :cases where great vprecision 1of pulse repetition .rates is fdesired, .the multivibrator oscillator 2| may' be coupled to .some standard Vfrequency source v.of sinusoidal current, lsuch as aV pulse .:commutatcr 22. This commutator 1s-.com-

. 9 prised of five multivibrators A, B, C, D, and E of a special kind which spend much more time in one temporarily stable unbalanced condition than Yin the other. -In the present example, each oscillator will operate at a frequency of 25,000 cycles but will spend four times as much time with current through one anode circuit than is spent with current through the other anode circuit. This inequality of time spent in one condition or the other is obtained by suitable selection and adjustment of the resistances and condensers associated with the tubes.

All of the iive multivibrator oscillators of the pulse commutator 22 Vof Fig. 5 are coupled together in such a manner that, going from left to right and then back to the beginning, each oscillator is synchronized by the one preceding. Thus, while all the oscillators are most of the time unbalanced in one direction there is a sort of wave of opposite unbalance running around the five oscillators in their cascaded or closed loop coupling arrangement. The speed of this wave is determined rst by the circuit constants in the circuits of the commutatcr and then more exactly by oscillator 2| in such a manner that each successive cycle of oscillation in oscillator 2I causes flipping of a next succeeding multivibrator around the coupled group of oscillators 22.

As a consequence of the successive nip-hopping of the oscillators in pulse commutator 22, the 'i pulse delay modulators 23, 24, 25, and 26 are each supplied with successive pairs of pulses which mark the beginning and end of the time intervals assigned to each of the four corresponding transmitters. The nrst pulse of each pair, say to modulator 23, throws the modulator 23 to one condition of unbalance and the second pulse throws the unbalance back again, if the modulator circuit has not already restored or thrown itself back.

The multivibrator pulse delay modulator 23, however, is so adjusted that, in the absence of modulation, it restores or throws itself back at a time half way between the two pulses from commutator system 22. This throwback governs, or

provides, pulses delivered to magnetron transmit- If modulation frequency energy is coupled into modulator multivibrator 23, as shown, then the time taken by the circuit to restore or throw itself back is modulated accordingly and we thereby produce modulation of the timing of pulses from transmitter II. The other magnetron transmitters I2, I3, and I4 produce pulses which are similarly modulated.

An outstanding virtue of the arrangement of Fig. 5 is that none of the modulators 23, 24, 25, and 2B can deliver pulsesto cause operation of its particular transmitter until after itreceives the first one of the pair of pulses assigned to it from commutator 22, and similarly none can cause operation of its transmitter any later than the second one of the pair of pulses. Thus there is no possibility of pulses of any one channel falling into time periods reserved for another channel. Instead, if excessive modulation input is applied to any one channel, the peak modulation will be strictly limited to keep each pulse transmitter in its own assigned time period.' Thus, while over-modulation can result in distortion in each channel it cannot cause interference between channels. v In Fig; 5 the magnetron transmitters II, i2,

10 I3, and I4 are 'of Ythe type described in my United States Patent application' No.' 470,768, filed December 31,' 1942, now U. S; Patent 2,409,038, granted October 8, 1946. Only certain essential elements of these magnetrons have been shown Vfor the sake of simplification of illustration.' The eld coil for producing the magnetic eld has not been shown. The anode, it will be noted, is of the multi-target cavity resonator type, which is coupled to an output circuit by means of a loop in one of the cavity spaces. However, it should be understood that any otherytype 'of pulse transjmitter in which radio frequency output pulses are produced in response to input control pulses may be used. SuchY transmitters are well known in radar and similar systems;

Fig. 6 -'graphically shows the wave form of currents in the several vacuum tube circuits of Fig. 5 with respect to time, including the effect .of modulation, in order that the operation ofthe system of Fig. 5 may be better visualized.

The automatic radio frequency commutator indicated in Figs. 1 and 5 for coupling the several transmitters successively to the common transmission line to an antenna is illustrated in greater detail in Fig. 7. yIn this'arrangement the inductively coupled koutput lead of each magnetron is separated from the antenna transmission line TL byv a spark gaprG. Each coupling lead is adjusted in length to make it resonant for the open circuit condition of the spark gap.' VAs a result, when the magnetron is operating, a very high radio frequency potentialima'y'be built up across the spark gap, yif necessary, to cause the gap to arc over. This gap spark-over potential may be large compared with the potentials developed at any time on the loaded transmission line TL to the antenna. y

At the same time' that one magnetron is delivering pulse power through its particular spark gap, the othermagnetrons are inactive and, due to absence of electron space charge, are somewhat mistuned from thetransmitted frequency. As a consequence, the Ilead'to each inactive magnetron presents'a relatively high'impeda'nce at its spark gap and capacity Vcoupling across the gap tends tov build up potential at the end of the lead to reduce the potential across the spark gap. This, combined with the fact that the loaded transmission line does not rise to a very high potential while beingfed with power from any one magnetron tends to discourage sparking across rthe gaps Vassociated with the inactive magnetrons. Thus under suitable conditions l,of gap length, gas -pressure in the gaps, and circuit adjustments, only that gap in circuit with an active magnetron will be short-circuited and nearly all the pulse power output of an active magnetron will be delivered tothe antenna trans mission line f i For vstill more positive commutation, .obtainable with less care in adjustment, direct current pulse discharges may be"superimposed on the radio frequency ldischargesl in thespark gaps but this, of course, requires more equipment complication.

There are-many detail circuit arrangements which may be utilized to perform the functions indicated 'in the various units .of Fig. 4. One detail arrangement is shown in the schematic diagram of Fig. 8 in which the parts have been numbered to correspond with the numbering in Fig. 4.

Referring to Fig. 8, received signal power from the receiving antenna is Vdelivered over a transmission line TL to high frequency amplifier 29,

the transmission line being terminated in such a manner as 'to deliver substantially maximum power, Without reection '.of waves back toward the antenna. Ampliiier/ 29 has three main purposes. One is to raiseI upl the received signal power for obtaining an improvement in signalV to receiver noise ratio by reducing Vthe relative eiect of anode circuit noise in heterodyne converter .or detector 30. Another purpose is to reduce the probability of radiation from heterodyne oscillator 3| reaching other receivers and a third purpose is to protect the receiverV converter 3l) from the effects of interference from signals or noise far removed in frequency from the frequency of the desired signal.

Amplified signal output from amplifier 29 is combined with power from heterodyne oscillator 3l in the input to converter 3|)l Yas a result of which the power currents-beat together to projvide ,output at an intermediate or lower frequency which is supplied Vto intermediate frequency (1; F.) amplifier 32. I have shown only one stage of amplification in 32 but it should be understood that a number of stages may be used. The amplier 32 with its oneor more stages, raises the signal power to a relatively high level and limits the band of frequencies passedthrough it to a band only -wide enough to pass thelsignal pulses plus someadditional band width to allow for mistuning and frequency drift between transmitter and receiver.

Output from I. amplifier 32 is delivered to pulse rectier`33'where'the1I. F. carrier current pulses are converted into direct current. The pulse rectier has two diodesin series which are so biased that no current iiows in either diode until output from I. F. amplier 32- rises above the eifective receiver noise level. In practice I would expect to adjust the bias so that none, or only a relatively small number of noise peaks cause any rectication. Thus the' effect of receiver noise while no pulse is being received, is made zero or very small.

However, whengcurrents above the peak noise level appear in the output of amplifier 32 the upper one of the two diodes passes-rectified current and changes the control electrode potential of coupling amplifier 33a; Thus, signal pulses higher than the peaks of noisecause coupling amplier 33a to pass on rectied .or direct current pulses from its outputl to pulse demodulator 34.

If now the pulse power increases toa value much greater .than twicey the peak value .of receiver noise, the lowerofthe-.two diodes inl 33 and the control electrode in coupling tube 33a begin to pass current andto limitthepeak pulse potential passedfon by'rcoupling ampliiier 33a. Thus, so long as the Ysignal pulses substantially exceed twice the peak amplitude "of noise in the output of I. F. amplifier 32,the amplitude-of pulses delivered to demodulator 34 is substantially constant and almostcleaned of the'effect of noise excepton thev endsof thepulses.

A portion of therpulseoutputV from rectiiiers 33 is delivered to pulse Vselectorunit 3l. Pulse selector 3l is-a Vacuum tubewhch passes a pulse of anode :current at each pulse delivered to it; These pulses of anode current tend `to hold down the anodeA potential of-'the tube in pulse oscillator 38 but, during the time interval` of the missing pulse, illustrated in Fig. 2, the anode potential rises toa relativeiyhigher value and this results in :pulse oscillator 3l!v being synchronized by the missing pulse.

Pulse'os'cillator 33 thenpulses'at arate'of 25,000

Ypulses per second in synchronism with the'repeti'v Ycapable of responding to the synchronizing force of selector 3l exceptatV a rate' less" than the lowest modulation frequency.

Pulse outputfrom unit de, synchronized by the missing illustrated in Fig'. 2 is now passed through a manually adjustable pulse delayer 39. l.his is a type of multivibrator or flip-flop circuit combination which tendsA to remain permanently unbalanced in one direction until the balance is thrown to the other direction by pulses from. unit 33, After being thrown to the temporarily unbalanced direction or condition by the pulses from unit 38, the circuit 35i will restore itself suddenly to the normal direction or conditio-n after a time delay which is adjustable by adjusting the circuit constants. This adjustment is to he made by the operator, either by switching circuit elements in and out whichhav'e been preadjusted, or by rotating'a dial which changes the circuit constants. This time delay adjustment is the operators means for selecting or tuning in any one of thefour transmitters tothe exclu-Y sion oi the others.

When circuit 39 restores itself to its normal condition, it pulses a similar circuit 39o the func` tion of which is'to produce square Wave impulses of a length corresponding to the timeV periods assigned to each transmitter, in this case about 8 microseconds as illustrated in Fig. '3. This' is accomplished by letting short pulses from unit vthrow the .unbalance of circuit 39avfrom the permanently (i. e., normal) to the temporarily unbalanced condition andthen adjusting the circuit constants cf dto, until it automatically re-Y stores or throws itself back again after the desired 8 microsecond-tifme interval.

While unit 3sat is in its temporarily unbalanced condition it renders coupling tube amplifier 33a operative so that it will pass any pulses arriving during the selected 8 mlcrosecond time periods but reject or block all other pulses. Thus I de liver to demodulator 3d only those pulses produced by a selected one of the fourtran's'mitters of Fig.` 5.

At the same time, the beginning of the rectangular wave pulse from 39a into 33a throws the balance of demcdulator circuit 34 in one directie:L where it remains until a signal pulse from 33a throws it back again. Since the timingY of the signal pulse is varied by modulation it follows that the percentage of time spentby de- 13 the 8 microsecond time periods assigned to reception of a particular transmitter. This positive square wave of potential cannot reach the circuits of demodulator 34 because of the condenser shown in the coupling lead from'oscillator-Sa. to demodulator 34. However, the beginning of the positive potential delivered to 33a does cause a short positive pulse of potential to be delivered to demodulator 34 and this short pulse throws the balance of the flip-flop circuits of 34 to a condition such that theleft anode, as seen in Fig. 8, carries current and the right anode none.

Now, if a signal pulsecomes into the receiver, it will cause the rectiiiersk 33 to ldeliver a positive pulse of `potential toa control electrode of coupling tube amplifier 33a, causing it-to pass a pulse of anode current. The pulse of anode Acurrent will result in a relatively negative pulse of potential on the anode and this negative kpulse is delivered through a coupling condenser to demodulator 34. This causes the nip-flop circuit of' demodulator 34 to reverseiits unbalanced condition so that the right anode will carry current and the left anode will have zero current. This last unbalanced condition will continue until the oscillator 39a delivers a pulse to demodulator Vlll Iof a polarity to throw the balance again. Before this happens, oscillator 39a, will have delivered a negative pulse to demodulator 34, at the end of the square wave potential wave delivered to coupling tube amplifier 33a but this negative pulse will have no effect on the demodulator because the negative signal pulsealready will have thrown the balance. 1

Thus, the beginnings of square wave pulses from synchronized oscillator 39a throw the balance of demodulator 34 in one direction at constant time intervals and, in between,the signal pulses throw the balance back. The-signal pulses, although they are repeated at the same average rate as the pulses from 39a, are modulated in timing by the input to the transmitter which is being received. Therefore the time interval after oscillator 39a throws the balance of 34 before the signal pulse throws the balance back, is modulated. As a consequence, the average currents to the two anodes of demodulator 34 are varied or` modulated differentially, in accordance with the useful modulation. i

The average` currents, after filtering' to remove higher frequency components of variations, contain a differential A.C. component of current which is the useful modulation current and this is amplified in audio amplifier 35 to drive the loudspeaker. If desired, for best possible results, the oscillator 38 may be synchronized Vby pulse selector 31 through a long time constant synchronizingk system of the kind described by Wendt andi Redendall in an article entitled, Automatic frequency and phase control,y of synchronization in television receivers, Proceedings The Institute of Radio Engineers, vol. 31, No. 1, JanuaryY 1943. Another alternative scheme of operation is to operate the transmitter next preceding the blankV space of Fig. 2 normally without Vmodulation so that its pulses, followed by the blank space, more or less rigidly control the frequency and timing of oscillator 38. In this case, when the pulses of this normally unmcdulated transmitter are modulated the modulation will be heard in all the `receivers adjusted to reception of the other transmitters. Such a scheme of operation reserves the one transmitter for synchronizing and forl the' transmission of very important information which;

should be heard by'all listeners. In war time, for example, it may be utilized for air raid warning or for other purposes of an urgent nature which must reach a maximum number of people.

What is claimed is:

1. A radio broadcasting system comprising a plurality of radio broadcasting stations operating sequentially over a common wavel radiating structure and from different transmitters, each of said stations having a keyed radio frequency oscillation generator circuit adapted to transmit program modulated power in the form of short pulses of high frequency energy, a common source of constant frequency energy controlling the operation of said differentbroadcasting stations to cause them to transmit pulses of the same repetition rate, and a delay circuit between the keyed radio frequency circuit of each of said broadcasting stations and said common source, said delay circuits giving diiferent time delays for the different broadcasting stations, whereby the pulses'transmitted from the different stationsoccur at diiferent time intervals.

2. A radio broadcasting systemcomprising a plurality of radio broadcasting stations operating sequentially over a common antenna from difiere ent transmitters, each of said stations having Ya keyed radio frequency oscillation generator circuit adapted to transmit program modulatedV power in the form of short pulses ofhighfrequency energy, and a common constant frequency pulse oscillator controlling the operation of said different broadcasting stations to cause them 'to transmit pulses of the same repetition rate.

`3. A radio broadcasting system comprising `a plurality of radio broadcasting stations operating sequentially over a common wave radiating struc. ture and from different transmitters, each of said stations having a keyed radio frequency generatorY adapted to transmit program modulated power in the form of short pulses of high frequency energy, a pulse producing circuit for keying the radio frequency generator, and means for connecting the pulse producing circuits of all of said broadcasting stations to a common sourceY of sinusoidal Waves of constant frequency, said common source controlling the operation of said -pulse producing circuits to cause them to producepulses of the same repetition rate.

4. A radio broadcasting system comprising a plurality of radio broadcasting stations operating sequentially over a common wave radiating structure and from different transmitters, each of said stations having a keyed radio, frequency generator adapted to transmit program modulated power in the form of short pulses of high frequency energy, a pulse producing circuit for keying the radio frequency generator, means for connecting the pulse'producing circuitsof all of said broad-` casting stations to a common 'source of sinusoidal-y waves of constant frequency, said common source controlling the operation of said pulse producing circuits to cause them to produce pulses of the same repetition rate, and an adjustable time delay circuit associated with each pulse producingcircuit, said delay circuits being adjusted to give diiferent time delays for the different stations, whereby the pulses from the different stations occupy different time periods.

5. A radio broadcasting system comprising a` plurality of different radiok transmitters and serving the Vsame area, each of said transmitters having a keyed radio frequency generator adapted to transmit program modulated power in theform of pulses of high frequencyenergy whicharexshortl compared to the time intervals between them, a common source of constantv frequency energy for controlling the pulse rates of said different transmitters, the frequencyof said common source being appreciably higher than the highest modulation frequency', a common antenna, and means for periodicallyv assigning said common antenna to said transmitters on a time division basis.

6. A radio broadcasting system comprising a plurality of radio broadcasting transmitters which transmit individual programs byl means of pulses of high frequency energy which are short compared to the time intervals between them, each of said transmittters including a, radio frequency generator, the repetition rate of the pulses for the different; transmitters being the same, the pulses from the diierent. transmitters occupying different time periods, a common radiating circuit for all said stations, a transmission line section of predetermined length and a spark gap in series therewith for coupling each transmitter to said common radiating circuit only when said transmitter is providing pulse power output.

7. A radio broadcasting system comp-risinor a plurality of radio. broadcasting transmitters which transmit individual programs by means of pulses of ultra high frequency energy of substantially the same carrier frequency and which pulses are short compared to the time intervals between them, means coupled to said stations for assuring that the' repetition rate of the pulses for the difierent transmitters is the same but appreciablyl higher than the highest modulation frequency and that the pulses from. the different transmitters occupy different time periods, each of said transmittersl including a radio frequency generator, and means vfor modulating the timing of the pulses from said transmitters over a limited range while maintaining the amplitude and length of said pulses constant.

8. A radio broadcasting system comprising a plurality of .radio broadcasting transmitters which transmit individual programs by means of pulses of high frequency energy which are short compared to the time intervals between them, the repetition rate of the pulses for the different transmitters being the same, the pulses from the different transmitters occupying different time periods, a common radiating circuit for all of said transmitters, each of said transmitters including a radio frequency generator, means including individual transmission line sections of predetern minedlength and individual spark gaps in series with said sections for coupling any one of said transmitters to said common radiating circuit onlyv when said one transmitter is providing pulse power output, and means for superimposingmodulation upon the pulses from each transmitter in accordance with the intelligence to be transmitted.

9. A radio broadcasting system operating on the time division principle, comprising a plurality of radio broadcasting transmitters, each of said transmitters having a keyed radio frequency generator, means for applying speech modulation to each of said transmitters, whereby said transmittersV send out individual speech programs by means of pulses of ultra high frequency energy which are short compared to the time intervals between them, the repetition rate of the pulses for the different transmitters being the same but appreciably higher than the highest modulation frequency, a common load circuit for all of said transmitters, and means for causing the different transmitters to produce pulses occupying ldiiierent non-overlapping time periods, said meansV inclu-ding a pulse delay modulator for each transmitter upon which is impressed a particular speech program to be broadcast by its associated station, and electronic switching circuits for rendering said transmitters sequentially operative.

10. A radio broadcasting system comprising a plurality of radio broadcasting transmitters, means for applying' speech modulation to each of said transmitters, whereby said transmitters send out individual speech programs by means of pulses of substantially the same carrier frequency and which pulses are short compared to the time intervals between them, therepetition rate of the pulses for the different transmitters being the same but appreciabl'y higher than the highest modulation frequency, a common load circuit for all of said transmitters, and means for causing the diuerenttransmitters to produce pulses occupying different time periods, said means including a pulse delay modulator for each transmitter upon Whichis impressed a particular speech program to be broadcast by its associated station, and electronic switching circuits for rendering .said transmitters sequentially operative, and means including a transmission line and spark gap arrangement individual toeach transmitter tor couplinga transmitter to said common load circuit only when said transrriitterl is providing pulse power output.

11. The combination with a radio broadcasting system comprising a plurality of radio broadcasuv ing transmitters which transmit individual pro grams by means of pulses of high frequency energy of substantially the same carrier frequency and which pulses are short compared to the time in'- tervals between them, the repetition rate of the pulses from the different transmitters being the same, each transmitter including a radio frequency generator, and means for causing said transmitters to function periodically at different non-overlapping time intervals, a common wave radiating structure for said transmitters adapted to be eiectively coupled at different times to said transmitters; oi' a receiver which is responsive the repetition rate of the transmitted pulses but for time periods not exceeding the operating time periods assigned to each transmitter.

12. A communication system comprising a single antenna and a plurality of channels adapted to be operatively connected to said anftenna at diiierent non-overlapping times, each of said channels including a radio frequency generator for producing pulses of high frequency energy and telephonic equipment for modulating the timing of said pulses, the high frequency energy produced in all of said channels having substantially the same frequency, the pulses produced in each channel having the same repetition rate.

13. A communication system comprising a single antenna and a plurality of channels adapted to be operatively connected to antenna at different non-overlapping times, each of said channels including a radioV frequency gen erator for producing pulses of high frequency energy Whose duration is short compared to the time interval between them, the high frequency energy produced in all of said channels having substantially the same frequency, and telephonie equipment for modulating the timing of said pulses, the pulses produced in each channel hav-v ing the same repetition rate.

14. In a radio communication system, a plu-` rality of independent channels each having telephonic modulationequipment and a radio fre- 17 quency generator, the generators in said channels producing substantially theV same carrier ,"fre'- quency, afcommon scurce-pf,pulses for producing pulses whichfare shortcompared-to the time inn tervalsfb'etween them,v ine`ans-foi-l assigning 'the 'output from said source tothe generators in said 'channels-in succession, v11o/therebyv enable. thel Iproduction-of pulses of radio'frequency energ'y'inr said dierentchannels atv diffrent time intervals, a common load, A'and aradiofrequeney commutator for coupling the-radio frequencygenerators of loadat said different channels tosaid common` different non-overlapping times, f, ,-gj; i

15. A radio broadcasting system comprising a plurality of radio broadcasting stations operating over the same wave radiating structure and from different transmitters, each of said stations having a keyed radio frequency generator circuit adapted to transmit program modulated power in the form of short pulses of high frequency energy, the high frequency energy transmitted by the different stations having the same frequency, and a common source of constant frequency energy controlling the operation of said dilferent broadcasting stations to cause them to transmit pulses of the same repetition rate, and a common antenna for said different transmitters.

16. In a multiplex system comprising a plurality of channels sequentially assigned to the same transmission medium, said channels containing signals in the form of short pulses modulated in phase, means for selecting and demodulating the phase modulated pulses in one of said channels, said means including a nip-flop circuit having two conditions of stable unbalanced equilibrium, means to throw this circuit to one unbalanced condition at the beginning of repeated time periods assigned to the selected channel, means responsive to signal pulses to throw the circuit back to its other unbalanced condition after time intervals varied by the modulation of the chosen channel, and means responsive to the proportion of time spent by the circuit in one or the other of its two conditions for deriving modulation frequency currents.

17. A radio broadcasting system comprising a plurality of radio broadcasting transmitters, means for applying speech modulation to said transmitters, whereby said transmitters send out individual speech programs by means of .pulses which are short compared to the time intervals between them, each transmitter including a radio frequency generator, the repetition rate of the pulses for the different transmitters being the same but higher than the highest modulation frequency, means coupled to and controlling the transmitters to produce pulses which occupy different time periods, a common load circuit for said transmitters, and means for effectively coupling each of said transmitters to said load circuit only during the active pulse producing period of the transmitter.

18. A radio broadcasting system comprising a plurality of broadcasting stations transmitting pulses of ultra high frequency energy, each of said stations including a magnetron oscillation generator, pulse producing means cou-pled to the generators of said stations, and sequentially supplying pulses thereto for controlling said generators to produce pulses at the same repetition rate but at different non-overlapping time intervals, and a circuit individual to each of said first means for modulating a characteristic of the pulses employed to control the respective magnetron.

19. A radio broadcasting system comprising a of said generators.

plurality of broadcasting'"transmitters producing pulses of radiol frequencyene'rgyloi'fthesame carrier frequency, the pulses produced by each transmitterbeingshort compared to the. time intervals between adjacetpulssfeach offs'aid transmitters including a radio` frequency generator, means for causing the`I pulses from different transmitters Yto have the saine repetition,ratebutf-occurat different non-overlapping times, anda common 'wave radiating structure' vfor said transmitters.'

20. YA radio broadcasting" system comprising a plurality. of ,simultaneouslypperating broadcasting'.v sta'tioris,prcdiicing pulses of .high .frequency energy, means Vfor causingthe' pulses from said stations to have the same repetition rate butto occur at different non-overlapping times, means for modulating the timing of the pulses produced by each broadcasting station, and a common antenna for said stations.

21. A radio broadcasting system comprising a plurality of radio broadcasting transmitters having a common wave radiating structure, each of said transmitters having a radio frequency generator and means for keying said generator to produce .program modulated power in the form of short pulses of high frequency energy, and a common source of constant frequency energy for controlling the operation of the said different broadcasting transmitters to cause them to transmit pulses of the same repetition rate.

22. A radio broadcasting system comprising a plurality of broadcasting transmitters producing pulses of radio frequency energy at the same repetition rate, the pulses produced by each transmitter being short compared to the time intervals between adjacent pulses from the same transmitter, each of said transmitters including a radio frequency generator and means for modulating the time of the pulses produced by said transmitter, the pulses from the different transmitters occurring at different non-overlapping times, and a common wave radiating structure for said transmitters.

23. A radio time division broadcasting system comprising a plurality of radio broadcasting transmitters which transmit individual lspeech programs by means of pulses of substantially the same high frequency carrier and which pulses are short compared to the time intervals between them, the repetition rate of the pulses for the different transmitters being the same but at least twice as high as the highest modulation frequency, the pulses from the different transmitters occupying different non-overlapping time periods, each of said transmitters including a radio frequency generator and means for applying modulation representative of speech to the generator; and a common radio frequency current carrying line sequentially assigned to the outputs CLARENCE W. HAN SELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,319,068 Hammond, Jr. -..M Oct. 21, 1919 1,530,660 Friedman Mar. 24, 1925 1,573,303 Colpitts Feb. 16, 1926 1,802,745 Whitaker Apr. 28, 1931 1,969,328 Roosenstein Aug. 7, 1934 (Other references on following page) Number ,-11111 21 1936 ,Mayv 7, 1946 June 18, 1940 4116.211940 Rowssrem peca 11, 19.40 'n''vf Jiihe '24, 1941 Bttf 1I 16, 1941 Dlr'irie et 21, 11011.16, 1941 Dallas Y Nov. 1s, 1941 Nimm 2,266,401, 2,212,011 2,295,585 2,298.65?r 

